Method and apparatus for signaling image information, and decoding method and apparatus using same

ABSTRACT

The present invention relates to a method for signaling image information and to a decoding method using same. The method for signaling image information according to the present invention comprises: a step of performing inter-prediction for a current picture; and a step of signaling information including the result of said inter-prediction and reference picture information indicating reference pictures usable in said inter prediction, wherein said reference picture information contains pieces of picture order count (POC) information of said usable reference pictures. The POC information of said usable reference pictures in said reference picture information is configured such that POCs for the pictures existing before said current picture in terms of a POC sequence are located at the front, and POCs for the pictures existing after said current picture in terms of a POC sequence are located following the POCs located at the front.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase Application under 35 U.S.C.§371 of International Application PCT/KR2012/007614, filed on Sep. 21,2012, which claims the benefit of U.S. Provisional Application No.61/537,586, filed on Sep. 22, 2011, the entire content of the priorapplication is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to video compression techniques, and moreparticularly, to method and device for efficiently signaling videoinformation and decoding method and device using the same.

BACKGROUND ART

Recently, demands for a high-resolution and high-quality image haveincreased in various fields of applications. As an image has higherresolution and higher quality, an amount of information on the videoincreases more.

Accordingly, when video information is transferred using media such asexisting wired or wireless broadband lines or video information isstored in existing storage media, the information transfer cost and theinformation storage cost increase.

High-efficiency video compressing techniques can be used to effectivelytransfer, store, and reproduce information on high-resolution andhigh-quality images.

Inter prediction and intra prediction can be used to enhance videocompression efficiency. In the inter prediction, pixel values of acurrent picture are predicted with reference to information of otherpictures. In the intra prediction, pixel values of a current picture arepredicted using inter-pixel relationships in the same picture.

When the inter prediction is carried out, a video encoder and a videodecoder can perform a prediction process on the basis of a referencepicture list indicating reference pictures which can be used for acurrent block (current picture).

Information for constructing a reference picture list can be transmittedfrom the video encoder to the video decoder. The video decoder canconstruct a reference picture list on the basis of the informationreceived from the video encoder and can effectively perform the interprediction.

SUMMARY OF THE INVENTION Technical Problem

An object of the present invention is to provide method and device foreffectively signaling video information in encoding/decoding the videoinformation.

Another object of the present invention is to provide method and devicefor effectively signaling information for inter prediction inencoding/decoding video information.

Still another object of the present invention is to provide method anddevice for effectively signaling information for constructing areference picture list used to perform an inter prediction.

Still another object of the present invention is to provide method anddevice for effectively constructing a reference picture list used toperform an inter prediction on the basis of received information.

Solution to Problem

According to an aspect of the present invention, there is provided amethod of signaling video information, including the steps of performingan inter prediction on a current picture; and signaling informationwhich includes reference picture information indicating the result ofthe inter prediction and reference pictures available in the interprediction, wherein the reference picture information includes pictureorder count (POC) information elements of the available referencepictures, and wherein the POC information elements of the availablereference pictures in the reference picture information are arranged sothat POC values of pictures previous to the current picture in an POCorder are located in front, and POC values of pictures subsequent to thecurrent picture in the POC order are located subsequently.

At this time, the POC information elements may be arranged in a POCdescending order of reference pictures for the reference picturesprevious to the current picture in the POC order and may be arranged ina POC ascending order of reference pictures for the reference picturessubsequent to the current picture in the POC order.

The POC information elements may include a POC difference between atarget reference picture and another picture out of the referencepictures indicated by the reference picture information, and the POCinformation elements of the reference pictures may be arranged in thereference picture information on the basis of the POC value of thetarget reference picture.

The POC information elements of the reference pictures may include themagnitude and the sign of a POC difference between a target referencepicture and a criterion picture out of the reference pictures indicatedby the reference picture information, and the POC information elementsof the reference pictures in the reference picture information may bearranged on the basis of the POC values of the target referencepictures.

At this time, the POC information elements in the reference pictureinformation may be arranged in a POC descending order of referencepictures for the reference pictures previous to the current picture inthe POC order and may be arranged in a POC ascending order of referencepictures for the reference pictures subsequent to the current picture inthe POC order. When the target reference picture is any one of thepicture closest to the current picture out of the pictures previous tothe current picture in the POC order and the picture closest to thecurrent picture out of the pictures subsequent to the current picture inthe POC order in the reference pictures indicated by the referencepicture information, the criterion picture may be the current picture.When the target reference picture is not any one of the picture closestto the current picture out of the pictures previous to the currentpicture in the POC order and the picture closest to the current pictureout of the pictures subsequent to the current picture in the POC orderin the reference pictures indicated by the reference pictureinformation, the criterion picture may be the reference picturecorresponding to the POC information element immediately previous to thePOC information element of the target reference picture.

At this time, the sign of the POC difference may be a sign of adifference between the POC value of the target reference picture and thePOC value of the current picture.

The POC information elements available in the reference pictureinformation may include information indicates the magnitude of a POCdifference between a target reference picture and a criterion pictureout of the reference pictures indicated by the reference pictureinformation; the number of reference pictures when the sign of the POCdifference is negative; and the number of reference pictures when thesign of the POC difference is positive, and the POC information elementsof the reference pictures in the reference picture information may bearranged on the basis of the POC value of the target reference picture.

At this time, the sign of the POC difference may be a sign of adifference between the POC value of the target reference picture and thePOC value of the current picture.

In addition, the POC information elements in the reference pictureinformation may be arranged in a POC descending order of referencepictures for the reference pictures previous to the current picture inthe POC order and may be arranged in a POC ascending order of referencepictures for the reference pictures subsequent to the current picture inthe POC order. When the target reference picture is any one of thepicture closest to the current picture out of the pictures previous tothe current picture in the POC order and the picture closest to thecurrent picture out of the pictures subsequent to the current picture inthe POC order in the reference pictures indicated by the referencepicture information, the criterion picture may be the current picture.When the target reference picture is not any one of the picture closestto the current picture out of the pictures previous to the currentpicture in the POC order and the picture closest to the current pictureout of the pictures subsequent to the current picture in the POC orderin the reference pictures indicated by the reference pictureinformation, the criterion picture may be the reference picturecorresponding to the POC information element immediately previous to thePOC information element of the target reference picture.

According to another aspect of the present invention, there is provideda method of decoding video information, including the steps ofentropy-decoding information of a received bitstream and obtainingreference picture information including picture order count (POC)information elements of reference pictures available in prediction of acurrent picture; and performing a prediction on the current block usinga reference picture list which is constructed on the basis of POC valuesof the reference pictures derived from the reference pictureinformation, wherein the POC information elements of the availablereference pictures in the reference picture information are arranged sothat POC values of pictures previous to the current picture in an POCorder are located in front and POC values of pictures subsequent to thecurrent picture in the POC order are located subsequently.

The POC information elements in the reference picture information may bearranged in a POC descending order of reference pictures for thereference pictures previous to the current picture in the POC order.

The POC information elements in the reference picture information may bearranged in a POC ascending order of reference pictures for thereference pictures subsequent to the current picture in the POC order.

The i-th (where i is an integer) POC information element POCi in thereference picture information may be a POC information of a referencepicture Pi, the POCi may include the magnitude of a POC differencebetween the reference picture Pi and a criterion picture in thereference picture information, and the POC information elements of thereference pictures in the reference picture information may be arrangedon the basis of the POC values of the reference pictures.

At this time, the POC information elements in the reference pictureinformation may be arranged in a POC descending order of referencepictures for the reference pictures previous to the current picture inthe POC order and may be arranged in a POC ascending order of referencepictures for the reference pictures subsequent to the current picture inthe POC order. When the reference picture Pi is any one of the pictureclosest to the current picture out of the pictures previous to thecurrent picture in the POC order and the picture closest to the currentpicture out of the pictures subsequent to the current picture in the POCorder in the reference pictures indicated by the reference pictureinformation, the criterion picture may be the current picture. When thereference picture Pi is not any one of the picture closest to thecurrent picture out of the pictures previous to the current picture inthe POC order and the picture closest to the current picture out of thepictures subsequent to the current picture in the POC order in thereference pictures indicated by the reference picture information, thecriterion picture may be the reference picture corresponding to the POCinformation element immediately previous to the POC information elementof the target reference picture.

In addition, the POC information elements may include informationindicating the sign of a POC difference between the reference pictureand the current picture.

The i-th (where i is an integer) POC information element POCi in thereference picture information may be a POC information element of areference picture Pi, the POCi may include the magnitude of a POCdifference between the reference picture Pi and a criterion picture inthe reference picture information, and the POC information elements ofthe reference pictures in the reference picture information may bearranged on the basis of the POC values of the reference pictures.

At this time, the POC information elements in the reference pictureinformation may be arranged in a POC descending order of referencepictures for the reference pictures previous to the current picture inthe POC order and may be arranged in a POC ascending order of referencepictures for the reference pictures subsequent to the current picture inthe POC order. When the reference picture Pi is any one of the pictureclosest to the current picture out of the pictures previous to thecurrent picture in the POC order and the picture closest to the currentpicture out of the pictures subsequent to the current picture in the POCorder in the reference pictures indicated by the reference pictureinformation, the criterion picture may be the current picture. When thereference picture Pi is not any one of the picture closest to thecurrent picture out of the pictures previous to the current picture inthe POC order and the picture closest to the current picture out of thepictures subsequent to the current picture in the POC order in thereference pictures indicated by the reference picture information, thecriterion picture may be the reference picture corresponding to the POCinformation element immediately previous to the POC information elementof the target reference picture.

The reference picture information may include information indicating anorder relationship between the POC value of each reference picture andthe POC value of the current picture.

The POC information elements may include POC difference information andsign information. When the number of reference pictures previous to thecurrent picture in the POC order out of m reference pictures indicatedby the reference picture information is n, the POC difference value forthe reference picture k corresponding to the k-th (where 0≦k≦n−1) POCdifference information element POCk out of the POC differenceinformation may be a difference between the POC value of a firstcriterion picture and the POC information element POCk, and the POCdifference value of the reference picture j corresponding to the j-th(where n≦j≦m) POC difference information POCj out of the POC differenceinformation elements may be the sum of the POC value of a secondcriterion picture and the POC difference information element POCj.

At this time, the first criterion picture may be the current picturewhen k is equal to 0, the first criterion picture may be the referencepicture corresponding to the (k−1)-th POC difference information elementwhen k is not equal to 0, the second criterion picture may be thecurrent picture when j is equal to n, and the second criterion picturemay be the reference picture corresponding to the (n−1)-th POCdifference information element when j is not equal to n.

Advantageous Effects

According to the present invention, it is possible to effectively signalvideo information in encoding/decoding the video information.

According to the present invention, it is possible to effectively signalinformation for constructing a reference picture list used to perform aninter prediction.

According to the present invention, it is possible to reducetransmission overheads in transmitting information for constructing areference picture list.

According to the present invention, it is possible to effectivelyconstruct a reference picture list for inter prediction on the basis ofinformation received information with low complexity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically illustrating a video encoderaccording to an embodiment of the present invention.

FIG. 2 is a block diagram schematically illustrating a video decoderaccording to an embodiment of the present invention.

FIG. 3 is a diagram schematically illustrating examples of a candidateblock which can be used to perform an inter prediction on a currentblock.

FIG. 4 is a diagram schematically illustrating an example of a referencepicture set which is signaled from the video encoder to the videodecoder.

FIG. 5 is a diagram illustrating an example of a reference relationshipbetween B pictures on which bidirectional prediction is performed.

FIG. 6 is a diagram illustrating an example of a reference relationshipbetween a B picture and a P picture.

FIG. 7 is a flowchart schematically illustrating an encoding procedurewhich is performed by the video encoder according to the presentinvention.

FIG. 8 is a flowchart schematically illustrating a decoding procedurewhich is performed by the video decoder according to the presentinvention.

DESCRIPTION OF EMBODIMENTS

The invention may be variously modified in various forms and may havevarious embodiments, and specific embodiments thereof will beillustrated in the drawings and described in detail. However, theseembodiments are not intended for limiting the invention. Terms used inthe below description are used to merely describe specific embodiments,but are not intended for limiting the technical spirit of the invention.An expression of a singular number includes an expression of a pluralnumber, so long as it is clearly read differently. Terms such as“include” and “have” in this description are intended for indicatingthat features, numbers, steps, operations, elements, components, orcombinations thereof used in the below description exist, and it shouldbe thus understood that the possibility of existence or addition of oneor more different features, numbers, steps, operations, elements,components, or combinations thereof is not excluded.

On the other hand, elements of the drawings described in the inventionare independently drawn for the purpose of convenience of explanation ondifferent specific functions in a video encoder and a video decoder, anddo not mean that the elements are embodied by independent hardware orindependent software. For example, two or more elements out of theelements may be combined to form a single element, or one element may besplit into plural elements. Embodiments in which the elements arecombined and/or split belong to the scope of the invention withoutdeparting from the concept of the invention.

Hereinafter, exemplary embodiments of the invention will be described indetail with reference to the accompanying drawings. The same elements inthe drawings will be referenced by the same reference signs and thedescription of the same elements will not be repeated.

FIG. 1 is a block diagram schematically illustrating a video encoderaccording to an embodiment of the present invention. Referring to FIG.1, a video encoder 100 includes a picture splitting module 105, aprediction module 110, a transform module 115, a quantization module120, a rearrangement module 125, an entropy encoding module 130, adequantization module 135, an inverse transform module 140, a filtermodule 145, and a memory 150.

The picture splitting module 105 may split an input picture into atleast one process unit. Here, the process unit may be a prediction unit(hereinafter, referred to as a “PU”), a transform unit (hereinafter,referred to as a “TU”), or a coding unit (hereinafter, referred to as a“CU”).

As will be described later, the prediction module 110 includes an interprediction module that performs an inter prediction and an intraprediction module that performs an intra prediction. The predictionmodule 110 may perform a prediction on the process unit of the picturesplit by the picture splitting module 105 to generate a predicted block.The process unit of a picture in the prediction module 110 may be a CU,a TU, or a PU. It may be determined whether the prediction performed onthe corresponding process unit is an inter prediction or an intraprediction, and specific details (for example, a prediction mode) of theprediction methods may be determined. The process unit subjected to theprediction may be different from the process unit of which theprediction method and the specific details are determined. For example,the prediction method and the prediction mode may be determined in theunits of PU and the prediction may be performed in the units of TU.

In the inter prediction, a prediction may be performed on the basis ofinformation on at least one of a previous picture and/or a subsequentpicture of a current picture to generate a predicted block. In the intraprediction, a prediction may be performed on the basis of pixelinformation of a current picture to generate a predicted block.

A skip mode, a merge mode, a motion vector prediction (MVP), or the likemay be used as the intra prediction method. In the inter prediction, areference picture may be selected for a PU, and a reference block havingthe same size as the PU may be selected. The reference block may beselected in the unit of integer pixels. A predicted block may begenerated so that a residual signal from a current PU is minimized andthe magnitude of a motion vector is minimized.

The predicted block may be generated in the unit of integer pixelsamples or in the unit of pixel samples less than an integer pixel, suchas ½ pixel samples and ¼ pixel samples. Here, a motion vector may alsobe expressed in the unit of pixel samples less than an integer pixel.For example, luma pixels may be expressed in the unit of ¼ pixels andchroma pixels may be expressed in the unit of ⅛ pixels.

Information such as an index of a reference picture selected through theinter prediction, a motion vector (for example, a motion vectorpredictor), and a residual signal may be entropy-encoded and transmittedto a video decoder. When the skip mode is used, the predicted block maybe used as a reconstructed block and thus the residual signal may not begenerated, converted, quantized, and transmitted.

When the intra prediction is performed, a prediction mode may bedetermined in the unit of PUs and the prediction may be performed in theunit of PUs. Alternatively, a prediction mode may be determined in theunit of PUs and the intra prediction may be performed in the unit ofTUs.

The prediction modes in the intra prediction may include 33 directionalprediction modes and at least two non-directional modes. Thenon-directional modes may include a DC prediction mode and a planarmode.

In the intra prediction, a predicted block may be generated after afilter is applied to reference samples. At this time, it may bedetermined whether a filter should be applied to reference samples,depending on the intra prediction mode of a current block and/or thesize of the current block.

A PU may be a block having various sizes/shapes. For example, in case ofthe inter prediction, a PU may be a 2N×2N block, a 2N×N block, a N×2Nblock, or a N×N block (where N is an integer). In case of the intraprediction, a PU may be a 2N×2N block or a N×N block (where N is aninteger). The PU having a block size of N×N may be set to be used inonly a specific case. For example, the PU having a block size of N×N maybe set to be used for only a CU having the smallest size or may be setto be used for only the intra prediction. In addition to theabove-mentioned sizes, PUs such as a N×mN block, a mN×N block, a 2N×mNblock, and a mN×2N block (where m<1) may be additionally defined andused.

Residual values (a residual block or a residual signal) between thegenerated predicted block and the original block may be input to thetransform module 115. The prediction mode information, the motion vectorinformation, and the like used for the prediction may be encoded alongwith the residual values by the entropy encoding module 130 and may betransmitted to the video decoder.

The transform module 115 may perform a transform on the residual blockby transform units and generate transform coefficients. The transformunit in the transform module 115 may be a TU and may have a quad treestructure. The size of the transform unit may be determined within apredetermined range of largest and smallest sizes. The transform module115 may transform the residual block using a discrete cosine transform(DCT) and/or a discrete sine transform (DST).

The quantization module 120 may quantize the residual values transformedby the transform module 115 and may generate quantization coefficients.The values calculated by the quantization module 120 may be provided tothe dequantization module 135 and the rearrangement module 125.

The rearrangement module 125 may rearrange the quantization coefficientsprovided from the quantization module 120. By rearranging thequantization coefficients, it is possible to enhance the codingefficiency in the entropy encoding module 130. The rearrangement module125 may rearrange the quantization coefficients in the form of atwo-dimensional block to the form of a one-dimensional vector throughthe use of a coefficient scanning method. The rearrangement module 125may enhance the entropy encoding efficiency in the entropy encodingmodule 130 by changing the coefficient scanning order on the basis ofstochastic statistics of the coefficients transmitted from thequantization module.

The entropy encoding module 130 may perform an entropy encoding on thequantization coefficients rearranged by the rearrangement module 125.Examples of the entropy encoding method include an exponential golombmethod, a CAVLC (Context-Adaptive Variable Length Coding) method, and aCABAC (Context-Adaptive Binary Arithmetic Coding) method. The entropyencoding module 130 may encode a variety of information such asquantization coefficient information and block type information of a CU,prediction mode information, split unit information, PU information,transfer unit information, motion vector information, reference pictureinformation, block interpolation information, and filtering informationtransmitted from the rearrangement module 125 and the prediction module110.

The entropy encoding module 130 may give a predetermined change to aparameter set or syntaxes to be transmitted, if necessary.

The dequantization module 135 dequantizes the values quantized by thequantization module 120. The inverse transform module 140 inverselytransforms the values dequantized by the dequantization module 135. Theresidual values generated by the dequantization module 135 and theinverse transform module 140 may be merged with the predicted blockpredicted by the prediction module 110 to derive a reconstructed block.

FIG. 1 illustrates that the residual block and the predicted block areadded by an adder to generate a reconstructed block. Here, the adder maybe considered as a particular module (reconstructed block constructingmodule) that generates a reconstructed block.

The filter module 145 may apply a deblocking filter, an adaptive loopfilter (ALF), and a sample adaptive offset (SAO) to the reconstructedpicture.

The deblocking filter may remove a block distortion generated at theboundary between blocks in the reconstructed picture. The ALF mayperform a filtering on the basis of the resultant values of comparisonof the original picture with the reconstructed picture of which theblocks have been filtered by the deblocking filter and/or the SAO. TheALF may be applied only when high efficiency is necessary. The SAO mayreconstruct an offset difference of the residual block, which has beensubjected to the deblocking filter, from the original image in the unitof pixels and may be applied in the form of a band offset and an edgeoffset.

On the other hand, the filter module 145 may not perform a filtering onthe reconstructed block used in the inter prediction.

The memory 150 may store the reconstructed block or picture calculatedby the filter module 145. The reconstructed block or picture stored inthe memory 150 may be provided to the prediction module 110 thatperforms the inter prediction.

FIG. 2 is a block diagram schematically illustrating a video decoderaccording to an embodiment of the present invention. Referring to FIG.2, a video decoder 200 may include an entropy decoding module 210, arearrangement module 215, a dequantization module 220, an inversetransform module 225, a prediction module 230, a filter module 235, anda memory 240.

When a video bitstream is input from the video encoder, the inputbitstream may be decoded on the basis of the order in which videoinformation is processed by the video encoder.

For example, when the video encoder uses a variable length coding(hereinafter, referred to as “VLC”) method such as the CAVLC method toperform the entropy encoding, the entropy decoding module 210 mayimplement the same VLC table as the VLC table used in the video encoderand may perform the entropy decoding. When the video encoder uses theCABAC method to perform the entropy encoding process, the entropydecoding module 210 may perform the entropy decoding using the CABACmethod to correspond thereto.

Information for generating a predicted block out of the informationdecoded by the entropy decoding module 210 may be provided to theprediction module 230, and the residual values entropy-decoded by theentropy decoding module 210 may be input to the rearrangement module215.

The rearrangement module 215 may rearrange the bitstream entropy-decodedby the entropy decoding module 210 on the basis of the rearrangementmethod in the video encoder. The rearrangement module 215 mayreconstruct and rearrange coefficients expressed in the form of aone-dimensional vector into coefficients in the form of atwo-dimensional block. The rearrangement module 215 may be provided withinformation associated with the coefficient scanning performed by thevideo encoder and may perform the rearrangement using a method ofinversely scanning the coefficients on the basis of the scanning orderin which the scanning is performed by the video encoder.

The dequantization module 220 may perform dequantization on the basis ofthe quantization parameters provided from the video encoder and thecoefficient values of the rearranged block.

The inverse transform module 225 may perform the inverse DCT and/orinverse DST of the DCT and/or DST, which has been performed by thetransform module of the video encoder, on the quantization result fromthe video encoder. The inverse transform may be performed on the basisof a transfer unit or a split unit of a picture determined by the videoencoder. The transform module of the video encoder may selectivelyperform the DCT and/or DST depending on plural information elements suchas the prediction method, the size of a current block, and theprediction direction, and the inverse transform module 225 of the videodecoder may perform the inverse transform on the basis of the transforminformation on the transform performed by the transform module of thevideo encoder.

The prediction module 230 may generate a predicted block on the basis ofprediction block generation information provided from the entropydecoding module 210 and the previously-decoded block and/or pictureinformation provided from the memory 240.

When the prediction mode of a current PU is an intra prediction mode,the prediction module 230 may perform an intra prediction of deriving apredicted block on the basis of pixel information of a current picture.

When the prediction mode for a current PU is the inter prediction mode,the prediction module 230 may perform the inter prediction on thecurrent PU on the basis of information included in at least one of aprevious picture and a subsequent picture of the current picture. Atthis time, motion information for the inter prediction of the currentPU, for example, information on motion vectors and reference pictureindices, provided from the video encoder may be derived from a skipflag, a merge flag, and the like received from the video encoder.

The reconstructed block may be derived using the predicted blockgenerated by the prediction module 230 and the residual block providedfrom the inverse transform module 225. FIG. 2 illustrates that theresidual block and the predicted block are added by an adder to derive areconstructed block. Here, the adder may be considered as a particularmodule (reconstructed block constructing module) that generates areconstructed block.

When the skip mode is used, the residual signal may not be transmittedand the predicted block may be used as a reconstructed block.

The reconstructed block and/or picture may be provided to the filtermodule 235. The filter module 235 may perform a deblocking filtering, anSAO, and/or an ALF on the reconstructed block and/or picture.

The memory 240 may store the reconstructed picture or block for use as areference picture or a reference block and may provide the reconstructedpicture to an output module.

Encoded or decoded pictures may be stored in a memory such as a decodedpicture buffer (DPB). When a current picture is encoded or decoded, theprevious pictures stored in the DPB may be referred to for performing aprediction on the current picture.

Specifically, the video encoder and the video decoder may stored a listof the previously-encoded/decoded pictures as a reference picture listfor use in the inter prediction.

When the inter prediction is performed, the video encoder and the videodecoder may perform a prediction on a target block (current block) of acurrent picture by referring another picture. The inter prediction maybe performed by the prediction modes of the video encoder and the videodecoder as illustrated in FIGS. 1 and 2.

When the inter prediction is performed, as described above, theprediction may be performed on the current block using information ofavailable neighboring blocks adjacent to the current block. Here, theneighboring blocks may include a available block out of blocks locatedat the same position (co-located) with the current block in referencepictures, which can be referred to by the current block (hereinafter,the available block referred to as “Col block” (co-located block) forthe purpose of convenience of explanation).

A neighboring block used to perform a prediction on the current block inthe inter prediction is referred to as “candidate block” for the purposeof convenience of explanation.

In the inter prediction, the prediction may be performed on the currentblock on the basis of information of candidates blocks. In case of theskip mode or the merge mode, motion information (for example, motionvector) and/or a reference picture for a block selected from thecandidate blocks may be used as the motion information and/or thereference picture for the current block.

When the MVP is performed, the motion information (for example, motionvector) for a block selected from candidates blocks may be used as apredicted value of a motion vector for the current block, and thereference picture information for the current block may be transmittedfrom the video encoder to the video decoder. A motion vector difference(MVD) between the MVP derived from the candidate block and the motionvector for the current block may be transmitted from the video encoderto the video decoder, and the prediction module of the video decoder mayderive the motion information for the current block on the basis of theMVP and the MVD.

FIG. 3 is a diagram schematically illustrating an example of candidateblocks which can be used when the inter prediction is performed on acurrent block.

The prediction modules of the video encoder and the video decoder mayuse a block located at a predetermined position around a current block400 as a candidate block. For example, in the example of FIG. 3, twoblocks A₀ 410 and A₁ 420 located on the left-bottom side of the currentblock and three blocks B₀ 430, B₁ 440, and B₂ 450 located on theright-top side and the left-top side of the current block may beselected as spatial candidate blocks. In addition to the blocksspatially neighboring, the Col block 460 may be used as a temporalcandidate block.

At the time of performing the inter prediction, as described above, themotion information of the current block may use motion information of aselected block out of the neighboring blocks as it is or may be derivedon the basis of the motion information of the selected block out of theneighboring blocks.

On the other hand, regarding reference pictures used for the interprediction, reference pictures for the current blocks may be derivedfrom reference pictures of neighboring blocks or may be indicated byinformation received from the video encoder. In case of the skip mode orthe merge mode, the prediction module of the video decoder may use thereference pictures of the neighboring blocks as the reference picturesfor the current block. When the MVP is applied, the prediction module ofthe video decoder may receive the information indicating the referencepictures for the current block from the video encoder.

Pictures encoded/decoded previously to the current picture may be storedin a memory (for example, a decoded picture buffer (DPB)) and may beused for prediction of the current block (current picture). A list ofpictures which are available for the inter prediction of the currentblock may be stored as a reference picture list.

A P slice is a slice which is decoded through the intra prediction orthe inter prediction using at most one motion vector and a referencepicture. A B slice is a slice which is decoded through the intraprediction or the inter prediction using at most two motion vectors andtwo reference pictures. Here, reference pictures may include ashort-term reference picture and a long-term reference picture.

Reference picture list 0 (hereinafter, referred to as “L0” for thepurpose of convenience of explanation) is a reference picture list usedfor the inter prediction of a P slice or a B slice. Reference picturelist 1 (hereinafter, referred to as “L1” for the purpose of convenienceof explanation) is used for the inter prediction of a B slice.Therefore, L0 may be used for the inter prediction of a block of a Pslice using uni-directional prediction, and L0 and L1 may be used forthe inter prediction of a block of a B slice using bi-directionalprediction.

The video decoder may construct a reference picture list when a decodingis performed on a P slice and a B slice through the use of interprediction. A reference picture to be used for the inter prediction maybe indicated based on a reference picture list. A reference pictureindex is an index indicating a reference picture in the referencepicture list.

The reference picture list may be constructed on the basis of areference picture set transmitted from the video encoder.

The reference pictures constituting the reference picture list may bestored in a memory (for example, DPB).

The pictures (pictures encoded/decoded previously to a current picture)stored in the memory may be managed by the video encoder and the videodecoder. The video encoder and the video decoder store picturesnecessary for prediction of the current block and releases pictures notused for prediction of the current block from the memory.

When a sliding window method is used as the method of managing thereference pictures, the reference pictures may be managed by a simplemethod of releasing a reference picture in a predetermined time passesafter the reference picture is stored in the memory, but this method hasseveral problems. For example, since there is a reference picture whichis not available any more, the reference picture may not be releasedfrom the memory and thus efficiency may be lowered. Since a storedreference picture is released from the memory after a predeterminedtime, it may be difficult to manage long-term reference pictures.

A memory management command operation (MMCO) method of directlysignaling instruction information on management of reference picturesfrom the video encoder may be used in consideration of the problems ofthe sliding window method. However, even when the MMCO method is used, apicture loss may occur in the course of signaling. When a lost pictureincludes an MMCO command, the lost MMCO information may not bereconstructed and the memory (DPB) may not be maintained in a statewhere currently-necessary pictures are correctly managed. Therefore,there is a possibility that the inter prediction will be performedincorrectly.

In order to solve the above-mentioned problems, a method oftransmitting, at each of slice header, a list of reference picturesnecessary in the course of decoding a slice may be used. A kind ofabstract container including a list of reference pictures in the sliceheader may be referred to as “RefPicList”. Alternatively, as describedabove, in order to distinguish reference picture list 0 and referencepicture list 1 constructed by the video decoder, the list of referencepictures necessary in the course of decoding a slice may be referred toas a reference picture set.

A reference picture set or RefPicList (hereinafter, referred to as a“reference picture set” for the purpose of convenience of explanationand distinction from the reference picture list) may include referencepictures to be used for reference of a current picture/slice or a futurepicture/slice. For example, a reference picture set is informationtransmitted from a video encoder to the video decoder and picturesincluded in the reference picture set may be specified by picture ordercount (POC). The POC indicates a display order of a picture. At thistime, the POCs for the reference pictures in the reference picture setmay be relative POCs to the POC of a current picture.

The relative POC represents a POC difference between two pictures in thereference picture set. The relative POC of the reference pictureprevious to the current picture in the POC order (reference picturehaving a POC smaller than the POC of the current picture) is POCdifference from the immediately-previous reference picture in thereference picture set. The relative POC of the reference pictureprevious to the current picture in the POC order (reference picturehaving a POC larger than the POC of the current picture) is a POCdifference from the immediately-previous reference picture in thereference picture set. Here, in case of (1) the first reference picturein the reference picture set and (2) the reference picture having arelative POC of which the sign is different from that of the previousreference picture in the reference picture set, the magnitude of therelative POC is POC difference from the current picture.

The POC difference between two pictures in the reference picture set maybe expressed by the absolute value and the sign.

The reference picture set may be signaled from the video encoder to thevideo decoder for each P slice and for each B slice.

Reference picture lists L0 and L1 may be constructed on the basis of thereference picture set received from the video encoder or may beexplicitly transmitted from the video encoder.

When reference picture list L0 is constructed, among pictures having aPOC smaller than the POC of the current picture out of the pictures(pictures previous to the current picture in the POC order or thepictures of which the relative POC has a negative sign) and pictureshaving a POC larger than the POC of the current picture in the receivedreference picture set (pictures subsequent to the current picture in thePOC order or the pictures of which the relative POC has a positivesign), reference picture indices are first allocated to the POC smallerthan the POC of the current picture, whereby a reference picture list isconstructed.

For example, until all the reference picture indices constituting thereference picture list are allocated, (i) lower reference pictureindices are allocated to the pictures closer to the current picture inthe POC order, for the pictures having a POC smaller than the POC of thecurrent picture in the reference picture set for the currentpicture/slice, and then (ii) lower reference picture indices areallocated to the pictures closer to the current picture in the POCorder, for the pictures having a POC larger than the POC of the currentpicture in the reference picture set for the current picture/slice.

When reference picture list L1 is constructed, among pictures having aPOC smaller than the POC of the current picture out of the pictures(pictures previous to the current picture in the POC order or thepictures of which the relative POC has a negative sign) and pictureshaving a POC larger than the POC of the current picture in the receivedreference picture set (pictures subsequent to the current picture in thePOC order or the pictures of which the relative POC has a positivesign), reference picture indices are first allocated to the pictureshaving a POC larger than the POC of the current picture, whereby areference picture list is constructed.

For example, until all the reference picture indices constituting thereference picture list are allocated, (i) lower reference pictureindices are allocated to the pictures closer to the current picture inthe POC order, for the pictures having a POC larger than the POC of thecurrent picture in the reference picture set for the currentpicture/slice, and then (ii) lower reference picture indices areallocated to the pictures closer to the current picture in the POCorder, for the pictures having a POC smaller than the POC of the currentpicture in the reference picture set for the current picture/slice.

Here, the short-term reference pictures are exemplified, but in case ofa reference picture list including long-term reference pictures,reference picture lists L0 and L1 may be first subjected to theprocesses of (i) and (ii) and then pictures transmitted as the long-termreference pictures through the reference picture set may be addedthereto.

In this specification, a method of constructing a reference picture setof short-term reference pictures and constructing a reference picturelist will be described below. A reference picture in the belowdescription may mean a short-term reference picture.

At this time, in order to reduce the number of bits of the signaledreference picture set and to reduce complexity of the process ofconstructing a reference picture list in the video decoder, thereference pictures (information of the reference pictures, for example,POC values) in the reference picture set (a list of reference pictures)may be arranged and transmitted.

The reference pictures in the reference picture set are signaled in astate where (1) the reference pictures having a POC smaller than the POCof the current picture are arranged (ordered) in the beginning part ofthe reference picture set in the POC descending order, and then (2) thereference pictures having a POC larger than the POC of the currentpicture are arranged subsequently thereto in the POC ascending order.

For example, in the reference picture set, the reference pictures(information elements of pictures) having a POC smaller than the POC ofthe current picture are first arranged and then the reference pictures(information elements of pictures) having a POC larger than the POC ofthe current picture are arranged. Here, the arranged informationelements of the reference pictures may be the POCs of the referencepictures, or the relative POCs of the reference pictures, or themagnitudes and signs of the relative POCs of the reference pictures.

When the arranged information elements are the POCs of the referencepictures, the POCs of the reference pictures having a POC smaller thanthe POC of the current picture are arranged in the order getting apartfrom the POC of the current picture in the reference picture set andthen the POCs of the reference pictures having a POC larger than the POCof the current picture are arranged in the order getting apart from thePOC of the current picture.

When the arranged information elements are the relative POCs of thereference pictures, the relative POCs of the reference pictures having aPOC smaller than the POC of the current picture may be first arrangedand the relative POCs of the reference pictures having a POC larger thanthe POC of the current picture may be arranged subsequently thereto. Forexample, the relative POCs of the reference pictures having a POCsmaller than the POC of the current picture in the reference picture setare arranged in the POC order (descending order) of the referencepictures, and the relative POCs of the reference pictures having a POClarger than the POC of the current picture are subsequently arranged inthe POC order (ascending order) of the reference pictures. Here, therelative POC of the reference picture is difference value between thePOC of the current picture and the POC of the reference picture.

A relative POC in the reference picture set may be represented by themagnitude (absolute value) of the relative POC and the sign of therelative POC.

The sign of the relative POC of a reference picture indicates whetherthe reference picture is located previous or subsequent to the currentpicture in the POC order. Therefore, instead of transmitting informationspecifically indicating the sign (+ or −) of the relative POC, themagnitudes of the relative POCs of the reference pictures previous tothe current picture in the POC order may be first transmitted and themagnitudes of the relative POCs of the reference pictures subsequent tothe current picture may then be transmitted. The video decoder receivingthe reference picture set may determine that the magnitudes of therelative POCs first received are associated with the reference picturesprevious to the current picture in the POC order and the magnitudes ofthe relative POCs subsequently received are associated with thereference pictures subsequent to the current picture in the POC order.At this time, information indicating the number of reference picturesprevious to the current picture in the POC order and the number ofreference pictures subsequent to the current picture in the POC ordermay be transmitted with information on the magnitudes.

When the magnitudes of the relative POCs are transmitted, in thereference picture set, the magnitudes of the relative POCs of thereference pictures having a POC smaller than the POC of the currentpicture are arranged in the POC order (descending order) of thereference pictures, and the magnitudes of the relative POCs of thereference pictures having a POC larger than the POC of the currentpicture are arranged in the POC order (ascending order) of the referencepictures.

Table 1 shows an example of a method of determining the magnitudes andthe signs of the relative POCs in the video encoder.

TABLE 1   sign_ref_pic[i] = (ref_pic[i] − currentPOC > 0) ? + : − if(i > 0 && sign_ref_pic[i] == sign_ref_pic[i−1]), then, abs_ref_pic[i] =|ref_pic[i] − refValue|−|ref_pic[i−1] −refValue| else, abs_ref_pic[i] =|ref_pic[i]−refValue|

The video encoder may determine the magnitudes and the signs of therelative POCs of the reference pictures to be signaled through the useof the reference picture set using the method shown in Table 1.

Here, sign_ref_pic[i] specifies the sign of the relative POC for thei-th reference picture in the reference picture set. When the POC of thei-th reference picture (ref_pic[i]) is larger than the POC of thecurrent picture (currentPOC), the value of the sign sign_ref_pic[i] ofthe relative POC for the i-th reference picture indicates “+”. When thePOC of the i-th reference picture (ref_pic[i]) is not larger than thePOC of the current picture (currentPOC), the value of the signsign_ref_pic[i] of the relative POC for the i-th reference pictureindicates “−”.

abs_ref_pic[i] specifies the magnitude of the relative POC for the i-threference picture in the reference picture set. When the sign of therelative POC for the i-th reference picture is equal to the sign of therelative POC for the (i−1)-th reference picture, a difference valuebetween the difference between the POC of the i-th reference picture anda reference value (refValue) and the difference between the POC of the(i−1)-th reference picture and the reference value (refValue) is themagnitude of the relative POC for the i-th reference picture. That is,when the sign of the relative POC for the i-th reference picture isequal to the sign of the relative POC for the (i−1)-th referencepicture, the magnitude of the relative POC for the i-th referencepicture is a POC difference between neighboring reference pictures (adifference between the POC of the i-th reference picture and the POC ofthe (i−1)-th reference picture) in the reference picture set.

The reference value (refValue) is a POC value transmitted from the videoencoder or set in advance and is a reference POC value for deriving thefirst relative POC in the reference picture set. For example, thereference value (refValue) may be the POC value of the current picture.

The sign of the relative POC for the i-th reference picture may not beequal to the sign of the relative POC for the (i−t)-th referencepicture. This case is a case where the i-th reference picture in thereference picture set is the first picture or a case where the (i−1)-threference picture in the reference picture set is a picture previous tothe current picture in the POC order and the i-th reference picture is apicture subsequent to the current picture in the POC order. Therefore,when the sign of the relative POC for the i-th reference picture is notequal to the sign of the relative POC for the (i−1)-th referencepicture, the magnitude of the relative POC for the i-th referencepicture may be the difference between the POC of the i-th referencepicture and the reference value (refValue). Subsequently, since the signof the relative POC for the (i+1)-th reference picture is equal to thesign of the relative POC for the i-th reference picture, the magnitudeof the relative POC for the (i+1)-th reference picture is the differencebetween the POC of the (i+1)-th reference picture and the POC of thei-th reference picture.

The video encoder may transmit the magnitudes and the signs of therelative POCs for the reference pictures derived as described above asthe reference picture set. While, in transmitting the magnitudes of therelative POCs for the reference pictures, the video encoder may firsttransmit the magnitudes of the relative POCs for the reference picturesprevious to the current picture in the POC order and then may transmitthe magnitudes of the relative POCs for the reference picturessubsequent to the current picture. In this case, the video encoder maytransmit information indicating the number of reference pictures ofwhich sign of the relative POC is “−” (the numbers of pictures previousto the current picture in the POC order) and the number of referencepictures of which the sign of the relative POC is “+” (the number ofpictures subsequent to the current picture in the POC order).

Table 2 shows another example of a method of determining the magnitudesand the signs of the relative POCs in the video encoder.

TABLE 2 sign_ref_pic[i] = (ref_pic[i] − currentPOC > 0) ? + : − if (i >0 && sign_ref_pic[i] == sign_ref_pic[i−1]), then, abs_ref_pic[i] =|ref_pic[i] − currentPOC| − |ref_pic[i−1] − currentPOC| else,abs_ref_pic[i] = |ref_pic[i] − currentPOC|

In Table 2, a case that the reference value (refValue) is the POC valueof the current picture is explained as an example.

Similarly to Table 1, when the POC of the i-th reference picture(ref_pic[i]) is larger than the POC of the current picture (currentPOC),the value of the sign sign_ref_pic[i] of the relative POC for the i-threference picture indicates “+”. When the POC of the i-th referencepicture (ref_pic[i]) is not larger than the POC of the current picture(currentPOC), the value of the sign sign_ref_pic[i] of the relative POCfor the i-th reference picture indicates “−”.

When the sign of the relative POC for the i-th reference picture isequal to the sign of the relative POC for the (i−1)-th referencepicture, a difference value between the difference between the POC ofthe i-th reference picture and the POC of the current picture and thedifference between the POC of the (i−1)-th reference picture and the POCof the current picture is the magnitude of the relative POC for the i-threference picture. That is, when the sign of the relative POC for thei-th reference picture is equal to the sign of the relative POC for the(i−1)-th reference picture, the magnitude of the relative POC for thei-th reference picture is a POC difference between neighboring referencepictures (a difference between the POC of the i-th reference picture andthe POC of the (i−1)-th reference picture) in the reference picture set.

The sign of the relative POC for the i-th reference picture may not beequal to the sign of the relative POC for the (i−1)-th referencepicture. This case is a case where the i-th reference picture in thereference picture set is the first picture or a case where the (i−1)-threference picture in the reference picture set is a picture previous tothe current picture in the POC order and the i-th reference picture is apicture subsequent to the current picture in the POC order. Therefore,when the sign of the relative POC for the i-th reference picture is notequal to the sign of the relative POC for the (i−1)-th referencepicture, the magnitude of the relative POC for the i-th referencepicture may be the difference between the POC of the i-th referencepicture and the POC of the current picture. Subsequently, since the signof the relative POC for the (i+1)-th reference picture is equal to thesign of the relative POC for the i-th reference picture, the magnitudeof the relative POC for the (i+1)-th reference picture is the differencebetween the POC of the (i+1)-th reference picture and the POC of thei-th reference picture.

The video encoder may transmit the magnitudes and the signs of therelative POCs for the reference pictures derived as described above asthe reference picture set. While, in transmitting the magnitudes of therelative POCs of the reference pictures, the video encoder may firsttransmit the magnitudes of the relative POCs for the reference picturesprevious to the current picture in the POC order and then may transmitthe magnitudes of the relative POCs for the reference picturessubsequent to the current picture. In this case, the video encoder maytransmit information indicating the number of reference pictures ofwhich sign of the relative POC is “−” (pictures previous to the currentpicture in the POC order) and the number of reference pictures of whichthe sign of the relative POC is “+” (pictures subsequent to the currentpicture in the POC order).

The video decoder may receive information on the reference picture setfrom the video encoder and may construct or reconstruct the referencepicture set on the basis of the received information.

Table 3 shows an example of a method of recovering the information(POCs) of the reference pictures in the video decoder receiving thereference picture set.

TABLE 3   if (i > 0 && sign_ref_pic[i] == sign_ref_pic[i−1]), then,  if(sign_ref_pic[i] == −), then   ref_pic[i] = refValue − Σabs_ref_pic[i] else,   ref_pic[i] = refValue + Σabs_ref_pic[i] else,  if(sign_ref_pic[i] == −), then   ref_pic[i] = refValue − abs_ref_pic[i] else,   ref_pic[i] = refValue + abs_ref_pic[i]

The video decoder may reconstruct the POCs available in prediction ofthe current block (picture) using the method shown in Table 3 on thebasis of the reference picture information (the magnitude of therelative POC or the magnitude and the sign of the relative POC) receivedfrom the video encoder.

The POC of the i-th reference picture (ref_pic[i]) in the referencepicture set may be reconstructed on the basis of the magnitude(abs_ref_pic[i]) and the sign (sign_ref_pic[i]) of the relative POC forthe i-th reference picture.

As shown in Table 3, the video decoder may explicitly receive the signof the relative POC and may recover the POC of the reference picture.

When the sign of the i-th reference picture and the sign of the (i−1)-threference picture in the reference picture set are equal to each otherand the sign of the i-th reference picture is “−”, the POC of the i-threference picture is a value obtained by subtracting the sum of therelative POCs from the initial reference picture (0-th referencepicture) to the i-th reference picture from the reference value(refValue). When the sign of the i-th reference picture and the sign ofthe (i−1)-th reference picture in the reference picture set are equal toeach other and the sign of the i-th reference picture is “+”, the POC ofthe i-th reference picture is a value obtained by adding the sum of therelative POCs from the initial reference picture (0-th referencepicture) to the i-th reference picture to the reference value(refValue).

Here, the reference value (refValue) is a POC value transmitted from thevideo encoder or set in advance and is a reference POC value forderiving the first relative POC in the reference picture set. Forexample, the reference value (refValue) may be the POC value of thecurrent picture.

When the sign of the i-th reference picture in the reference picture setis not equal to the sign of the (i−1)-th reference picture, this casemeans a case where the i-th reference picture in the reference pictureset is the first picture or a case where the (i−1)-th reference picturein the reference picture set is a picture previous to the currentpicture in the POC order and the i-th reference picture is a picturesubsequent to the current picture in the POC order.

When the sign of the i-th reference picture and the sign of the (i−1)-threference picture in the reference picture set are not equal to eachother and the sign of the i-th reference picture is “−”, the POC of thei-th reference picture is a value obtained by subtracting the relativePOC for the i-th reference picture from the reference value (refValue).When the sign of the i-th reference picture and the sign of the (i−1)-threference picture in the reference picture set are not equal to eachother and the sign of the i-th reference picture is “+”, the POC of thei-th reference picture is a value obtained by adding the relative POCfor the i-th reference picture to the reference value (refValue).

Unlike in the example shown in Table 2, the information indicating thesigns of the relative POCs for the reference pictures may not beexplicitly transmitted. In this case, the video decoder may determinethat the signs of the relative POCs located in front in the referencepicture set are “−” (minus) and that the signs of the relative POCslocated subsequently in the reference picture set is “+” (plus). At thistime, information indicating the number of relative POCs of which thesign is “−” and the number of relative POCs of which the sign is “+” maybe transmitted from the video encoder. The video decoder may determinethat the relative POCs corresponding to the number of relative POCshaving a sign of “−” indicated by the video encoder from the beginningof the reference picture set have a sign of “−”, may determine that therest of the relative POCs have a sign of “+”, and may recover the POC ofthe i-th reference picture (ref_pic[i]) as described above.

In other words, the relative POC for the first reference picture amongthe reference pictures in the reference picture set is the POCdifferences from the reference value (refValue). The relative POC forthe picture, which is previous to the current picture, other than thefirst reference picture out of the reference pictures in the referencepicture set is the POC difference from the immediately-previousreference picture. The relative POC for the first subsequent picture tothe current picture out of the reference pictures in the referencepicture set is the POC difference from the POC of the current picture.The relative POCs for the other reference pictures in the referencepicture set (from the second reference picture subsequent to the currentpicture to the final reference picture in the reference picture set) arethe POC differences from the immediately-previous reference picture.Here, it is determined on the basis of the POC order whether a referencepicture is previous or subsequent to the current picture. Theimmediately-previous reference picture means a picture immediatelyprevious in the order in the reference picture set.

Table 4 shows another example of a method of recovering the information(POCs) of the reference pictures in the video decoder receiving thereference picture set.

TABLE 4   if (i > 0 && sign_ref_pic[i] == sign_ref_pic[i−1]), then,  if(sign_ref_pic[i] ==−), then   ref_pic[i] = currentPOC −(abs_ref_pic[i] + abs_ref_pic[i−1])  else,   ref_pic[i] = currentPOC +(abs_ref_pic[i] + abs_ref_pic[i+1]) else,  if (sign_ref_pic[i] ==−),then   ref_pic[i] = currentPOC − abs_ref_pic[i]  else,   ref_pic[i] =currentPOC + abs_ref_pic[i]

In the method shown in Table 4, it is assumed that the number ofreference pictures included in the reference picture set is 2 and thereference value (refValue) for deriving the first relative POC value inTable 3 is the POC of the current picture, in order to clearly describethe features of the present invention.

The video decoder may explicitly receive the signs of the relative POCsfrom the video encoder and may recover the POCs of the referencepictures.

When the sign of the i-th reference picture and the sign of the (i−1)-threference picture in the reference picture set are equal to each otherand the sign of the i-th reference picture is “−”, the POC of the i-threference picture is a value obtained by subtracting the relative POCfor the i-th reference picture and the relative POC for the (i−1)-threference picture from the POC of the current picture. When the sign ofthe i-th reference picture and the sign of the (i−1)-th referencepicture in the reference picture set are equal to each other and thesign of the i-th reference picture is “+”, the POC of the i-th referencepicture is a value obtained by adding the relative POC for the i-threference picture and the relative POC for the (i−1)-th referencepicture to the POC of the current picture.

When the sign of the i-th reference picture in the reference picture setis not equal to the sign of the (i−1)-th reference picture, this casemeans a case where the i-th reference picture in the reference pictureset is the first picture or a case where the (i−1)-th reference picturein the reference picture set is a picture previous to the currentpicture in the POC order and the i-th reference picture is a picturesubsequent to the current picture in the POC order. In this case, therelative POC for the i-th reference picture may be derived on the basisof the POC of the current picture as shown in Table 4.

Therefore, when the sign of the i-th reference picture and the sign ofthe (i−1)-th reference picture in the reference picture set are notequal to each other and the sign of the i-th reference picture is “−”,the POC of the i-th reference picture is a value obtained by subtractingthe relative POC for the i-th reference picture from the POC of thecurrent picture. When the sign of the i-th reference picture and thesign of the (i−1)-th reference picture in the reference picture set arenot equal to each other and the sign of the i-th reference picture is“+”, the POC of the i-th reference picture is a value obtained by addingthe relative POC for the i-th reference picture to the POC of thecurrent picture.

As described with reference Table 3, the information indicating thesigns of the relative POCs of the reference pictures may not beexplicitly transmitted. In this case, the video decoder may determinethat the signs of the relative POCs located in front in the referencepicture set are “−” (minus) and that the signs of the relative POCslocated subsequently in the reference picture set is “+” (plus). At thistime, information indicating the number of relative POCs of which thesign is “−” and the number of relative POCs of which the sign is “+” maybe transmitted from the video encoder. The video decoder may determinethat the relative POCs corresponding to the number of relative POCshaving a sign of “−” indicated by the video encoder from the beginningof the reference picture set have a sign of “−”, may determine that therest of the relative POCs have a sign of “+”, and may recover the POC ofthe i-th reference picture (ref_pic[i]) as described above.

In other words, the relative POCs for the first reference picture amongthe reference pictures in the reference picture set is the POCdifference from the reference value (refValue). The relative POCs forthe pictures, which are previous to the current picture, other than thefirst reference picture out of the reference pictures in the referencepicture set are the POC differences from the immediately-previousreference picture. The relative POCs for the first subsequent picture tothe current picture out of the reference pictures in the referencepicture set is the POC difference from the POC of the current picture.The relative POCs for the rest of reference pictures in the referencepicture set (from the second reference picture subsequent to the currentpicture to the final reference picture in the reference picture set) arethe POC differences from the immediately-previous reference picture.Here, it may be determined on the basis of the POC order whether areference picture is previous or subsequent to the current picture. Theimmediately-previous reference picture means a picture immediatelyprevious in the order in the reference picture set.

Examples of the present invention when the reference value (refValue) isthe POC of the current picture will be described specifically below.

FIG. 4 is a diagram schematically illustrating an example of a referencepicture set which is signaled from the video encoder to the videodecoder. In the example illustrated in FIG. 4, 9 P slices (P pictures)(P0 to P9) subjected to uni-directional prediction refer each other.

Table 5 shows an example where a reference picture set to be signaled inthe example illustrated in FIG. 4 includes POCs of reference pictures.

TABLE 5 current picture P0 P1 P2 P3 P4 P5 P6 P7 P8 POC of the currentpicture 20 21 22 23 24 25 26 27 28 reference picture set i = 1 19 20 2122 23 24 25 26 27 (POC) i = 2 16 19 20 21 20 23 24 25 24 i = 3 16 16 2020 20 24

Referring to FIG. 4 and Table 5, the reference picture set of thecurrent picture includes the POCS of the reference pictures which can bereferred to for the current picture. Regarding the reference pictures inthe reference picture set, lower indices are allocated to the referencepictures closer to the current picture in the POC order.

For example, in the example of FIG. 4 and Table 5, when the currentpicture is P6 (POC=26), the reference pictures which can be referred tofor the current picture are P5, P4, and P0. Accordingly, the referencepicture set of the current picture (P6) of which the POC is 26 includesthe POCS of P5, P4, and P0, and lower indices are allocated to thereference pictures closer to the current picture in the POC order.

In the example of FIG. 4 and Table 5, the reference pictures previous tothe current picture in the POC order are arranged in the descendingorder in the reference picture set, but the POCs of the referencepictures are directly signaled through the reference picture set.

Unlike this, the relative POCs for the reference pictures may besignaled through the reference picture set as described above.

Table 6 shows an example of the reference picture set to be signaled inFIG. 4, where the reference picture set includes the relative POCs forthe reference pictures.

TABLE 6 current picture P0 P1 P2 P3 P4 P5 P6 P7 P8 POC of the currentpicture 20 21 22 23 24 25 26 27 28 reference picture set i = 1 19 20 2122 23 24 25 26 27 (POC) i = 2 16 19 20 21 20 23 24 25 24 i = 3 16 16 2020 20 24 reference picture set i = 1 1 1 1 1 1 1 1 1 1 (magnitude of i =2 3 1 1 1 3 1 1 1 3 relative POC) i = 3 3 4 1 3 4 1 reference pictureset i = 1 − − − − − − − − − (sign of relative i = 2 − − − − − − − − −POC) i = 3 − − − − − −

In Table 6, the reference picture set of the current picture in FIG. 4is expressed by the POCs of the reference pictures, the magnitude of therelative POCs for the reference pictures, and the signs of the relativePOCs for the reference pictures.

Table 5 shows the case where the POCs of the reference pictures aredirectly transmitted, but Table 6 shows a case where the relative POCsfor the reference pictures are transmitted through the reference pictureset.

The relative POCs for the reference pictures previous to the currentpicture in the POC order (the reference pictures having a POC smallerthan the POC of the current picture) are POC differences from theimmediately-previous reference picture in the reference picture set. Thesigns of the relative POCs transmitted through the reference picture setindicate whether the corresponding reference picture is a pictureprevious or subsequent to the current picture in the POC order.

For example, in the example of FIG. 4 and Table 6, in consideration of acase where the current picture P5, the pictures which can be referred tofor the current picture are P4, P3, and P0 and the POCs thereof are 24,23, and 20.

When the relative POCs are transmitted through the reference picture setfor P5, the reference picture set in which the magnitudes and the signsof the relative POCs for the reference pictures of P5 are arranged inpredetermined orders is transmitted. As described above, in the exampleillustrated in FIG. 4 illustrating the reference relationship betweenthe P slices, the reference pictures are pictures previous to thecurrent picture in the POC order and the reference pictures in thereference picture set are arranged in the descending order.

Therefore, the magnitudes of the relative POCs in the reference pictureset for P5 are arranged in the order of P4, P3, and P0. As shown inTable 6, the magnitude of the relative POC for P4 is 1 and the signthereof is “−”, the magnitude of the relative POC for P3 is 1 and thesign thereof is “−”, and the magnitude for the relative POC of P0 is 3and the sign thereof is “−”, which are transmitted through the referencepicture set for P5.

Here, regardless of the signs of the relative POCs, a reference pictureset in which the reference pictures (the magnitudes of the relative POCsfor the reference pictures) previous to the current picture in the POCorder are arranged in front in the reference picture set and thereference pictures (the magnitudes of the relative POCs for thereference pictures) subsequent to the current picture are arrangedsubsequently in the reference picture set may be transmitted. In thiscase, information indicating the number of reference pictures previousto the current picture in the POC order (reference pictures of which thesign of the relative POC is “−”) and the number of reference picturessubsequent to the current picture in the POC order (reference picturesof which the sign of the relative POC is “+”) may be transmittedtogether with information on the magnitudes.

FIG. 5 is a diagram illustrating an example of a reference relationshipbetween B pictures subjected to bi-directional prediction, unlike inFIG. 4 illustrating the reference relationship between P picturessubjected to uni-directional prediction. FIG. 5 schematicallyillustrates the reference relationship among 9 B pictures B0 to B8.

Table 7 shows an example of a reference picture set to be signaled inFIG. 5, where a reference picture set includes relative POCs ofreference pictures.

TABLE 7 current picture B0 B1 B2 B3 B4 B5 B6 B7 B8 POC of the currentpicture 20 21 22 23 24 25 26 27 28 reference picture set i = 1 12 20 2022 20 24 24 26 20 (POC) i = 2 10 18 18 20 18 22 22 24 18 i = 3 8 22 2424 28 26 28 28 16 i = 4 4 24 28 28 18 12 reference picture set i = 1 8 12 1 4 1 2 1 8 (magnitude of i = 2 2 2 2 2 2 2 2 2 2 relative POC) i = 32 1 2 1 4 1 2 1 2 i = 4 4 2 4 4 2 4 reference picture set i = 1 − − − −− − − − − (sign of relative i = 2 − − − − − − − − − POC) i = 3− + + + + + + + − i = 4 − + + + + −

In the example of Table 7 and FIG. 5, instead of directly transmittingthe POCs of the reference pictures through the reference picture set,the relative POCs for the reference pictures may be transmitted throughthe reference picture set.

The relative POCs for the reference pictures previous to the currentpicture in the POC order (the reference pictures having a POC smallerthan the POC of the current picture) are POC differences from theimmediately-previous reference picture in the reference picture set. Therelative POCs for the reference pictures subsequent to the currentpicture in the POC order (the reference pictures having a POC largerthan the POC of the current picture) are POC differences from theimmediately-previous reference picture in the reference picture set.Here, the magnitudes of the relative POCS for (1) the first referencepicture in the reference picture set and (2) the reference picture ofwhich the sign of the relative POC is different from that of thereference picture previous in the reference picture set are POCdifferences from the current picture. In other words, the relative POCsfor the reference picture closest to the current picture out of thereference pictures previous to the current picture in the POC order inthe reference picture set and the reference picture closest to thecurrent picture out of the reference pictures subsequent to the currentpicture are POC differences from the current picture.

An example where the current picture is B5 will be described withreference to Table 7. The reference picture set includes B4, B2, B6, andB8. When the reference picture set includes the relative POCs, themagnitude of the relative POC to which the lowest index is allocated inthe reference picture set is associated with B4 and is 1 which is adifference value between the POC of the current picture and the POC ofB4, and the sign thereof is “−”. The magnitude of the relative POC towhich the second index is allocated is associated with B2 and is 2 whichis a difference value between the POC of B4 and the POC of B4, and thesign thereof is “−”. The magnitude of the relative POC to which thesecond index is allocated is associated with B6. Since the sign of therelative POC for B6 is different from the sign of the relative POC forB2 which is the previous reference picture, the magnitude of therelative POC for B6 is 1 which is a POC difference from the currentpicture and the sign thereof is “+”. The magnitude of the relative POCto which the final index is allocated is associated with B8 and 2 whichis a difference between the POC of B6 and the POC of B8 and the signthereof is “+”.

As described above, by transmitting only the magnitudes of the relativePOCs of the reference pictures to the current picture instead oftransmitting all the magnitudes and the signs of the relative POCs ofthe reference pictures to the current, and transmitting the magnitudesof the relative POCs having a sign of “−” earlier than the magnitudes ofthe relative POCs having a sign of “+”, the sign of the correspondingrelative POC may be derived without explicitly transmitting the signs.In this case, information indicating the number of relative POCs havinga sign of “−” and the number of relative POCs having a sign of “+” maybe transmitted together with information on the magnitudes.

For example, by considering the case where the current picture is B5 inTable 7 again, the video encoder may transmit the reference picture setof B5 including only the magnitudes of the relative POCs of thereference pictures such as (1 2 1 2). As shown in Table 7, themagnitudes of the relative POCs having a sign of “−” are located infront in the reference picture set. The arrangement order is thedescending order as described above for the relative POCs having a sign“−” (the relative POCs for the reference pictures previous to thecurrent picture in the POC order), and the ascending order as describedabove for the relative POCs having a sign of “+” (the relative POCs forthe reference pictures subsequent to the current picture in the POCorder). At this time, the information indicating the number of relativePOCs having a sign of “−” and the number of relative POCs having a signof “+” may be transmitted along with the reference picture set. Forexample, it is assumed that an indication that the number of referencepictures (relative POCs) having a sign of “−” in the reference pictureset is 2 and the number of reference pictures (relative POCs) having asign of “+” is 2 is received. Then, since the sign of the two previousrelative POCs in the reference picture set is “−” and the sign of twosubsequent relative POCs is “+”, the video decoder may determine thatthe magnitudes of the two previous relative POCs in the referencepicture set are the magnitudes of the relative POCs for the referencepictures having a POC smaller than the POC of the current picture andthe magnitudes of the two subsequent relative POCs in the referencepicture set are the magnitudes of the relative POCs for the referencepictures having a POC larger than the POC of the current picture.

FIG. 6 is a diagram schematically illustrating an example of a referencerelationship between a B picture and a P picture.

FIG. 6 illustrates the reference relationship between 7 P pictures P0 toP6 subjected to uni-directional prediction and two B pictures B0 and B1subjected to bi-directional prediction.

Table 8 shows an example of a reference picture set to be signaled inFIG. 6, where a reference picture set includes relative POCs ofreference pictures.

TABLE 8 current picture P0 P1 P2 B0 P3 P4 B1 P5 P6 POC of the currentpicture 20 21 22 23 24 25 26 27 28 reference picture set i = 1 19 20 2122 23 24 24 26 27 (POC) i = 2 16 19 20 21 20 23 20 25 24 i = 3 16 16 2420 28 24 reference picture set i = 1 1 1 1 1 1 1 2 1 1 (magnitude of i =2 3 1 1 1 3 1 4 1 3 relative POC) i = 3 3 4 1 3 2 1 reference pictureset i = 1 − − − − − − − − − (sign of relative i = 2 − − − − − − − − −POC) i = 3 − − + − + −

Table 8 and FIG. 6 are associated with cases where P pictures and Bpictures are mixed, but the method of inducing the magnitudes and thesigns of the relative POCs, the method of arranging the relative POCs ina reference picture set, and the like are the same as described above.

For example, when the current picture is B1, the reference picture setfor B1 includes relative POCs for P3, P0, and P6. The reference pictureset includes the magnitudes (2 4 2) and the signs of the relative POCsof P3, P0, and P6 and may be transmitted to the video decoder.

In this case also, instead of transmitting information indicating thesigns of the relative POCs, information indicating the number ofrelative POCs having a sign “−” and the number of relative POCs having asign of “+” on the basis of the arrangement order may be transmittedtogether with the reference picture set including the magnitudes of therelative POCs. For example, when the current picture is B1, thereference picture set (2 4 2) including the magnitudes of the relativePOCs and the information indicating that the number of relative POCshaving a sign of “−” is 2 and the number of relative POCs having a signof “+” is 1 may be transmitted.

FIG. 7 is a flowchart schematically illustrating an encoding procedurewhich is performed by a video encoder according to the presentinvention. The video encoder performing the encoding procedureillustrated in FIG. 7 corresponds to the video encoder described abovewith reference to FIG. 1.

Referring to FIG. 7, the video encoder performs a prediction on acurrent block (S710). The video encoder may perform inter prediction orintra prediction on the current block. When the inter prediction isperformed, reference pictures of the current block may beselected/designated using a reference picture list constructed asdescribed above.

The video encoder transforms/quantizes the prediction result on thecurrent block (S720). The video encoder may transform/quantize aresidual block corresponding to a difference between the predictionresult and the original block. When the intra prediction is performed,information on the performed intra prediction mode may betransformed/quantized. When the inter prediction is performed, motioninformation (information on motion vectors/reference pictures) may betransformed/quantized.

The video encoder entropy-encodes the transformed/quantized information(S730). The CABAC may be used as the entropy encoding method.

The video encoder signals the entropy-encoded information (S740). Atthis time, the signaled information includes a reference picture set forconstructing a reference picture list for the current picture (thecurrent block). The reference picture set may be constructed for eachslice and may be transmitted in a state where it is included in theslice header.

The reference picture set may include POCs of reference pictures for thecurrent block. The reference picture set may include relative POCs forthe reference pictures to reduce transmission overhead.

When the reference picture set is includes the relative POCs for thereference pictures, the magnitudes and the signs of the relative POCsfor the pictures available as the reference pictures of the currentpicture may be transmitted through the reference picture set, and thenumber of relative POCs having a sign of “−” and the number of relativePOCs having a sign of “+” along with the magnitudes of the relative POCsmay be transmitted. When the relative POCs are transmitted, the relativePOCs having a sign of “−” are first transmitted and the relative POCshaving a sign of “+” are then transmitted. The relative POCs having asign of “−” may be arranged in the descending order on the basis of thePOCs of the reference pictures, and the relative POCs having a sign of“+” may be arranged in the ascending order on the basis of the POCs ofthe reference pictures.

While the operations of the video encoder are schematically describedwith reference to FIG. 7 so as to easily understand the presentinvention in consideration of the details of the reference picture set,this is for convenience of explanation and the operations of the videoencoder according to the present invention may include the operationsdescribed with reference to FIG. 1.

FIG. 8 is a flowchart schematically illustrating a decoding procedurewhich is performed by a video decoder according to the presentinvention.

Referring to FIG. 8, the video decoder receives a bitstream from thevideo encoder and performs an entropy decoding (S810). The bitstreamreceived from the video encoder may include a reference picture set. Thereference picture set may be received in a state where it is included inthe slice header.

The reference picture set may include POCs of reference pictures for acurrent block or may include relative POCs for the reference pictures.

The video decoder may receive information indicating pictures availableas the reference pictures of the current picture through the referencepicture set. For example, the POCs of the pictures available as thereference pictures may be received through the reference picture set.When the reference picture set includes (1) the magnitudes and the signsof the relative POCs of the reference pictures or includes (2) themagnitudes of the relative POCs for the reference pictures and thenumbers of relative POCs having a sign of “−” and a sign of “+”, the POCof the corresponding reference picture may be derived using the methodshown in Table 4 on the basis of the received information.

When the relative POCs are received through the reference picture set,the relative POCs having a sign of “−” are first received and therelative POCs having a sign of “+” are then received. The relative POCshaving a sign of “−” are arranged in the descending order based on thePOCs of the reference pictures, and the relative POCs having a sign of“+” may be arranged in the ascending order based on the POCs of thereference pictures.

The video decoder may perform a prediction on the current block on thebasis of the entropy-decoded information (S820). The prediction methodfor the current block may be transmitted from the video encoder. Whenthe prediction method for the current block is inter prediction, thevideo decoder may perform the prediction using the reference picturelist constructed on the basis of the received reference picture set.

The method of constructing the reference picture list using thereference picture set is the same as described above. The constructedreference picture list may be stored in the memory of the video decoder.

The video decoder reconstructs a picture (S830). The video decoder mayreconstruct the current block on the basis of the prediction on thecurrent block and may reconstruct a picture (image) using thereconstructed blocks. When a skip mode is used, the residual signal isnot transmitted and thus the predicted block may be used as areconstructed block. When a merge mode or an MVP mode is used, the videodecoder may reconstruct the current block by adding the predicted blockand the residual block.

In this description, terms such as a “picture included in a referencepicture set” and an “x-th picture in a reference picture set” are used,but these terms are intended for convenience of explanation. A picturein the reference picture set may be a picture of which the POCinformation is included in the reference picture set. The x-th picturein the reference picture set may be a picture of which the POCinformation element is arranged at the x-th position in the referencepicture set.

On the other hand, the reference relationship between picturesillustrated in FIGS. 4 to 6 does not consider a temporal level, but thisis for understanding of the invention and the present invention is notlimited to this configuration. The present invention may be similarlyapplied to cases where only pictures of levels lower than the currentpicture are referred to in consideration of the temporal level. In thiscase, the reference relations shown in Tables 5 to 8 may be changedaccordingly.

While the methods in the above-mentioned embodiments have been describedon the basis of the flowcharts as a series of steps or blocks, theinvention is not limited to the order of the steps and a certain stepmay be performed in an order other than described above or at the sametime as described above. The above-mentioned embodiments include variousexamples. Therefore, the invention includes all substitutions,corrections, and modifications belonging to the appended claims.

When it is mentioned above that an element is “connected to” or “coupledto” another element, it should be understood that still another elementmay be interposed therebetween, as well as that the element may beconnected or coupled directly to another element. On the contrary, whenit is mentioned that an element is “connected directly to” or “coupleddirectly to” another element, it should be understood that still anotherelement is not interposed therebetween.

The invention claimed is:
 1. A video decoding method performed by adecoding apparatus including at least one of a processor and a memory,the method comprising: receiving, by the processor, picture order count(POC) information for configuring a reference picture set includingreference pictures which are prior to the current picture in a decodingorder; deriving, by the processor, POC values of the reference picturesin the reference picture set based on the POC information; configuring,by the processor, the reference picture set based on the POC values ofthe reference pictures; performing, by the processor, an interprediction on a current block based on the reference picture set toderive a predicted sample of the current block; and generating, by theprocessor, a reconstructed sample based on the predicted sample, whereina POC value of an i-th reference picture is derived based on a POCdifference specified by the POC information, wherein the POC informationspecifies the POC difference between the current picture and the i-threference picture if the i is equal to 0, and wherein the POCinformation specifies the POC difference between the i-th referencepicture and an (i−1)-th reference picture if the i is greater than
 0. 2.The method of claim 1, wherein the step of obtaining the POC informationfor configuring the reference picture set includes deriving, by theprocessor, POC differences for reference pictures previous to thecurrent picture in a POC order, in a descending order of the POC order,and deriving, by the processor, the POC differences for the referencepictures subsequent to the current picture in the POC order, in anascending order of the POC order.
 3. The method of claim 2, wherein POCdifferences for the reference pictures previous to the current picturein the POC order have a negative sign, and wherein POC differences forthe reference pictures subsequent to the current picture in the POCorder have a positive sign.
 4. The method of claim 1, wherein the POCinformation for configuring the reference picture set includes a firstnumber information for specifying the number of reference picturesprevious to the current picture in a POC order and a second numberinformation for specifying the number of reference pictures subsequentto the current picture in the POC order, wherein the step of obtainingthe POC information for configuring the reference picture set includesderiving, by the processor, information elements for specifying the POCdifferences of the reference pictures previous to the current picture inthe POC order by the number specified by the first number information,and deriving, by the processor, information elements for specifying thePOC differences of the reference pictures subsequent to the currentpicture in the POC order by the number specified by the second numberinformation.
 5. The method of claim 1, further comprising: deriving, bythe processor, a reference picture list based on an order of thereference pictures in the reference picture set, wherein the interprediction is performed based on the reference picture list derived fromthe reference picture set.
 6. The method of claim 5, wherein the orderof the reference pictures in the reference picture set includes an orderof a descending order of POC values for the reference pictures previousto the current picture in the POC order out of the reference pictures inthe reference picture set, and an order of an ascending order of POCvalues for the reference pictures subsequent to the current picture inthe POC order out of the reference pictures in the reference pictureset.
 7. The method of claim 6, wherein indices are allocated based onthe descending order of POC values to the reference pictures previous tothe current picture in the POC order and then indices are allocatedbased on the ascending order of POC values to the reference picturessubsequent to the current picture in the POC order when the referencepicture list is a reference picture list
 0. 8. The method of claim 6,wherein indices are allocated based on the ascending order of POC valuesto the reference pictures subsequent to the current picture and thenindices are allocated based on the descending order of POC values areallocated to the reference pictures previous to the current picture inthe POC order when the reference picture list is a reference picturelist
 1. 9. The method of claim 5, wherein the step of deriving thereference picture list includes deriving, by the processor, thereference picture list on the basis of a first order based on adescending order of POC values of the reference pictures previous to thecurrent picture in the POC order out of the reference pictures in thereference picture set; and a second order based on an ascending order ofPOC values of the reference pictures subsequent to the current picturein the POC order out of the reference pictures in the reference pictureset, wherein indices are allocated to the reference pictures previous tothe current picture based on the first order, indices are allocated tothe reference pictures subsequent to the current picture based on thesecond order, and then indices are allocated to long-term referencepictures when the reference picture list is a reference picture list 0,and wherein indices are allocated to the reference pictures subsequentto the current picture based on the second order, indices are allocatedto the reference picture previous to the current picture based on thefirst order, and then indices are allocated to long-term referencepictures when the reference picture list is a reference picture list 1.10. The method of claim 1, wherein POC the information for constructingthe reference picture set includes information for specifying POCdifference between a criterion picture and the reference picture, andwherein the step of obtaining the POC information for constructing thereference picture set includes obtaining, by the processor, a first POCinformation for specifying the POC differences of the reference picturesprevious to the criterion picture in the POC order and then obtaining,by the processor, a second POC information for specifying the POCdifferences of the reference pictures subsequent to the criterionpicture in the POC order.
 11. The method of claim 1, wherein the POCinformation for constructing the reference picture set is transmitted ata level of slice header.